[llvm] Handle backedge-count logic for std::reverse like loops (PR #92560)

[via llvm-commits]

mrdaybird wrote:

@efriedma-quic I copy-pasted the return logic because otherwise I had to push everything into a else branch(which would be pain to review and git-diff :sweat_smile:)


https://github.com/llvm/llvm-project/pull/92560